Microporous membrane containing interconnected voids is currently manufactured by using as a starting material a pre-formed organic polymer. As the first step, the polymer is dissolved in a suitable solvent or combination of solvents. After this solution has been filtered and debubbled, it is taken through steps which involve forming a wet film of the polymer solution, contacting this film with a nonsolvent for the polymer and then removing the nonpolymeric components in such a way that the desired final dry microporous membrane properties are obtained. The microporosity results from the phase separation when the polymer chains physically agglomerate to form the walls and the void volume results from those spaces in which the phase-separated solvent remains. At a further stage in the process this solvent is removed by exchange with water and the film is then dried. Through the choice of polymer, solvent and nonsolvent and by carefully controlling the process kinetics of phase separation caused by the diffusive entrance and exit of solvents and nonsolvents to and from the as yet-liquid film of polymer solution, the total void content and average void size (pore size) can be controlled. Since the steps which control pore size and porosity involve diffusion through a partially solidified swollen gel, the residence time required for a given set of conditions is usually in the order of several minutes. Thus the rate of production is at most about thirty linear feet per minute, and involves large machine path lengths of up to one hundred feet. However, the real limitations of the method are much more fundamental, and relate to the problems involved in the solution processing of polymers. To begin with, the method is restricted to polymers which are conveniently soluble in a limited number of water soluble solvent-nonsolvent systems. Since the solution properties and the mechanical properties of the polymer are quite sensitive to the original molecular weight distribution, batch to batch processability is very dependent on the consistency of the raw material. Such consistency can be difficult to attain, when, as is often the case, it is necessary to blend different lots of the polymer to make a given batch. Indeed, in large reactors even the attainment of a truly uniform solution of the polymer, or mixture in a combination of solvents or nonsolvents, can be difficult and time consuming, especially since shear-degradation due to mixing can change the intrinsic polymer properties. Further, the prepared polymer solutions are often prone to phase separation at room temperature. It is frequently a requirement that they be handled and maintained at elevated temperature during further processing, which is a further complication. In short, the current and previously used methods of manufacturing microporous membrane from preformed polymers are troublesome, time consuming, lend themselves only to a low rate of production and hence are expensive.
It is known, and indeed common practice, to manufacture porous polymeric material (foam) using monomers and oligomers as the starting materials and causing the polymerization reaction therebetween to occur simultaneously with the generation of voids. In such cases the void volume is generated from gases added to the formulation or formed during the reaction which lead to the formation of gas bubbles and which cause expansion of the polymerizing liquid. If the bubbles touch one another and sufficient pressure is generated, the walls between the bubbles become thin and collapse creating an "open celled" foam. The most common example of this is the manufacture of open-celled polyurethane. However, the cell size or pore size distribution attained, is very broad and usually far greater than 15 microns.
It is known to produce microporus polypropylene membrane by a process involving the swelling of a polypropylene film in a solvent and thereafter bi-axially stretching the film while in the swollen state. However, this process creates elliptically shaped voids in the membrane, which renders it mechanically fragile, and the total void content cannot be made to exceed about 30%. Hence the method is quite restrictive both as to the polymer used and as to the properties of the membrane formed.
It is also known to manufacture microporous laminate sheets of polytetrafluoroethylene on a backing fabric, such method involving the sintering of the tetrafluoroethylene polymer which is used as the starting material. Here again, however, the method is quite restrictive both as to the polymer which can be used and as to the properties of the final product.
In the art of polymer formation broadly, it is well known that electron beam or ultraviolet irradiation can be used to initiate the rapid polymerization of monomers or oligomers to a solid polymer. Ultraviolet radiation is more commonly used, in which case the starting material must include not just the monomer or oligomer but also a photoinitiator. Such method P-335 is currently in common use for example in the printing, textile, floor covering, and adhesive industries for rapidly forming polymeric coatings. One of the advantages to the method as currently used for forming polymeric coatings is that it does away with the need for conventional solvents. That is, the article on which the polymeric coating is desired can be coated with undiluted oligomer, plus a photoinitiator if required, and then irradiated to provide the polymeric coating. This saves the expense not only of the otherwise required solvent itself but also the expense of safely evaporating or otherwise removing it from the applied coating as required with more conventional coating methods. Such radiation induced polymeric coatings and the numerous monomers and oligomers as well as the photoinitiators and radiation sources which are useful in forming same are well described in existing patents and other literature as exemplified by the following:
Polymer News: PA1 UV Curing: Science and Technology, Editor S. P. Pappas, Technology Marketing Corp,. 1978. PA1 U.S. Pat. Nos.:
Vol. 4, No. 4, February 1978 (P. 175); PA2 Vol. 4, No. 5, April 1978 (P. 239); PA2 Vol. 4, No. 6, June 1978 (P. 268); PA2 Vol. 5, No. 1, September 1978 (P. 36); PA2 Vol. 5, No. 2, November 1978 (P.53); PA2 Vol. 5, No. 6, July 1979 (P. 283); PA2 Vol. 6, No. 6, July 1980 (P. 265); PA2 4,039,414 (Aug. 2, 1977) to SCM Corp. PA2 4,048,036 (Sept. 13, 1977) to PPG PA2 4,075,366 (Feb. 21, 1978) to DeSoto PA2 4,148,987 (April 10, 1979) to Rohm and Haas
British Pat. No. 932,126 teaches the manufacture of porous ion-exchange resins by subjecting to ultraviolet radiation a mixture of unsaturated monomers dissolved in a liquid which functions as a precipitant for the ion-exchange resin resulting from the polymerization of the monomers.